Phase-locked loop circuits incorporate a voltage controlled oscillator (VCO) in which the output of the VCO is fed back through a frequency divider as a divide-by-N circuit to an input of a phase detector, i.e., phase comparator. A constant frequency reference signal is applied to the phase detector from a reference source such as a crystal oscillator. A reference frequency divider is often used to reduce the frequency of the reference signal to the same frequency as the voltage controlled oscillator and/or impart a desired channel spacing. The frequency of the output signal from the voltage controlled oscillator can be changed by changing the ratio “N” of the divide-by-N circuit in a predetermined manner.
These types of frequency synthesizers are commonly used in communications systems, including commercial television systems, which often require the use of wideband low phase noise signals for television systems such as disclosed in commonly assigned U.S. Pat. Nos. 6,281,935 and 6,335,766, the disclosures which are hereby incorporated by reference in their entirety. These systems require the frequency of a transmitted signal to be controlled such that the frequency of the signal received at a transmitter is error corrected before being broadcast as a television signal. A pilot frequency signal is typically added to the signal to be broadcast after the signal has been shaped and filtered. The frequency of the RF signal is controlled by compensating for errors in the signal received from the system. Typically, a digital television transmitter for such systems uses an exciter circuit that receives a digital data stream corresponding to digital television signals and formats the digital data stream and converts the formatted digital stream into analog form in accordance with digital television standards. A power amplifier includes a mixer and modulates the digital television signals in analog form with a carrier frequency. A frequency control circuit controls the frequency of the bandwidth of the modulated carrier signals.
One possible drawback of such systems is the carrier on an adjacent channel could be off by a certain amount, for example, three (3) Hz and the pilot signal must be contained within that drift. It is desirable to lock at the frequency to obtain the final frequency such as by using a phase-locked loop and voltage controlled oscillator.
To generate wideband low phase noise signals is difficult because as the bandwidth is increased, the “Q” of the resonator is lowered to increase the pull range. As the “Q” is lowered, however, the phase noise of the generated signal is increased. Some prior art solutions have band split the frequencies so that a single voltage controlled oscillator does not cover the whole frequency band. This is costly, however, and requires a lot of board space and requires switching and multiple voltage controlled oscillators and phase-locked loops with the associated circuits such as loop filters, power supply filters and associated components for the phase-locked loop and voltage controlled oscillators and other components. There have been some techniques to lower phase noise by splitting the entire frequency band into smaller bands such that the voltage controlled oscillator does not cover a wide bandwidth using a band split phase-locked loop.